1. Field of the Invention
This invention relates to a novel sulfonium salt suitable for use in a chemically amplified, positive resist composition. It also relates to a chemically amplified, positive resist composition which is highly sensitive to high energy radiation such as deep-ultraviolet lights, electron rays and X-rays, can be developed with alkaline aqueous solution to form a pattern, and is thus suitable for use in a fine patterning technique. It also relates to novel bis(p-tert-butoxyphenyl)sulfoxide used as an intermediate in synthesizing the sulfonium salts.
2. Prior Art
As the LSI technology tends toward higher integration and high speed, further refinement of pattern rules is required. The current patterning technology mostly relies on light exposure which is now approaching to the essential limit of resolution which is dictated by the wavelength of a light source. It is generally recognized that in light exposure using g-line (wavelength 436 nm) or i-line (wavelength 365 nm) as a light source, a pattern rule of about 0.5 .mu.m is the limit. For LSIs fabricated by such light exposure technique, a degree of integration equivalent to 16 mega-bit DRAM is the limit. At present, LSIs fabricated in the laboratory have reached this stage. It is urgently required to develop a finer patterning technique.
Under such circumstances, deep-ultraviolet lithography is regarded promising as the next generation of fine patterning technology. The deep-UV lithography is capable of working on the order of 0.3 to 0.4 .mu.m. If a less light absorbing resist is used, it is possible to form a pattern having a side wall nearly perpendicular to the substrate. Great attention is now paid to the technique of utilizing a high illuminance KrF excimer laser as a deep-UV light source. In order to employ this technique on a mass production scale, a resist material having low light absorption and high sensitivity is desired.
From this point of view, a number of chemically amplified, positive working resist materials were recently developed using acid catalysts as disclosed in JP-B 27660/1990, JP-A 27829/1988, U.S. Pat. Nos. 4,491,628 and 5,310,619. These materials have high sensitivity, resolution and dry etching resistance and are promising as resist materials especially suited for deep-UV lithography.
It is known that the function of chemically amplified, positive resist materials is largely affected by photo-acid generators used therein. Typical photo-acid generators are onium salts as shown below. ##STR1##
The onium salts themselves are oil soluble. When blended as a resist component, they act to reduce the solubility of the resist material in alkaline aqueous solution and to prevent the resist film from thinning upon development.
However, in exposed areas of positive resist material, photo-acid generators absorb high energy rays to decompose into products which are also oil soluble. This reduces the rate of dissolution of the exposed areas in alkaline aqueous solution, failing to provide a high ratio of the alkali dissolution rate of exposed areas to that of unexposed areas (which ratio is known as dissolution contrast). Consequently, chemically amplified, positive resists using onium salts as mentioned above are low in resolution upon alkaline development, that is, poor in removal of exposed areas, resulting in a pattern cross-sectional shape being upward tapered in trapezoid instead of a rectangular shape.
Prior art chemically amplified, positive resists, however, suffer from the problem known as post-exposure delay (PED) that when deep-UV, electron beam or X-ray lithography is carried out, line patterns would have a T-top configuration, that is, patterns become thick at the top if the leave-to-stand or delay time from exposure to post-exposure baking (PEB) is extended. This problem, which arises probably because the resist surface is reduced in solubility, becomes a serious drawback on practical application. This not only makes difficult dimensional control in the lithographic process, but also adversely affects dimensional control in the processing of substrates using dry etching. In this regard, reference is made to W. Hinsberg et al., J. Photopolym. Sce. Technol., 6 (4), 535-546 (1993) and T. Kumada et al., J. Photopolym., Sci. technol., 6 (4), 571-574 (1993). There are available no chemically amplified, positive resists which can resolve this problem and are thus practically acceptable.
It is understood that basic compounds in the air largely participate in the PED problem associated with chemically amplified, positive resists. Light exposure generates acids at the resist surface which react with basic compounds in the air and are thereby deactivated. As the delay time until PEB is extended, more amounts of acids are deactivated and accordingly, decomposition of acid labile groups are more unlikely to occur. As a consequence, an insolubilized layer is formed at the resist surface, resulting in a T-top configured pattern.
It is known from JP-A 232706/1993 and 249683/1993 that since addition of a basic compound suppresses the influence of basic compounds in the air, it is also effective for resolving the PED problem. However, the basic compound used therein is little taken into the resist film due to volatilization, less compatible with resist components, and unevenly dispersible in a resist film over its width. Thus the basic compound cannot achieve its advantages in a reproducible manner and causes a drop of resolving power.